Thanks Kerim & FvM for all this help - I very much appreciate it! I've spent a lifetime in software and I'm really only starting to get to grips with hardware, and is there a lot to learn!!
I can give that a try, and there's only one ground pin on the micro (pin 14 I think).
Can I ask, are you leaning towards thinking the problem is related to the micro rather than the current drawn by the SN75176AP? The noise seems to coincide with the frequency of the output driver being enabled and the transciever datasheet suggests approx 10mA difference in Icc between the output driver enabled and disabled.
I should also mention that the LED shown on the board (D1) is programmed to illuminate when *sending* data, so this flickers rapidly and exactly coincides with the noise. I took out the code that lights the LED to see if that was contributing to the noise, but it made no difference.
When the micro is running, and receiving data on the RS485 bus, but not replying, there is no noise from the amp.
Once again, thanks you both for the help.
Ol